Camera modules designed to capture images of objects that emit near-infrared (near-IR) light are used in automotive applications and endoscopy. For example, FIG. 1 depicts a lesion 190 in the field of view of an endoscope 110. A camera module 120 images lesion 190, which is located with respect to a coordinate system 198 that has directions x, y, and z. Camera module 120 includes an image sensor 130 and an imaging lens 100. Image sensor 130 includes a pixel array 132 upon which imaging lens 100 forms an image of lesion 190. Pixel array 132 has a width 132W and imaging lens 100 has an optical axis 100X.
Effectiveness of endoscope 110 depends on its ability to meet performance specifications at a wavelength and temperature range characteristic of its application. A wavelength range is, for example, the visible wavelength range (free-space wavelength λ0∈[0.40 μm, 0.65 μm]) or the near infrared wavelength range (λ0∈[0.60 μm, 1.0 μm]). A typical temperature range for the aforementioned applications is 20° C. to 60° C. Hence, the performance of a camera that includes lens 100 depends in part on imaging lens 100 being athermalized such that it forms a clear image on image sensor 132 within the temperature range associated with its application. For example, imaging lens 100 images light 192 propagating from a point 190P on lesion 190 to a point 102. Light 192 is for example near-IR light. Point 102 is located a field height x1 from optical axis 100X and a focal shift Δz1 from pixel array 132. Focal shift Δz1=0 corresponds to when imaging lens 100 forms an in-focus image of point 190P on image sensor 132. The magnitude of focal shift Δz1 as a function of field height x1 and ambient temperature T is an athermalization metric for lens 100. For a prior-art lens, at visible and near-IR wavelengths the on-axis focal length shifts by Δz1=9.2 μm between 20° C. and 60° C., or
            Δ      ⁢                          ⁢              z        1                    Δ      ⁢                          ⁢      T        =      0.23    ⁢                  ⁢    μm    ⁢          /        ⁢    °    ⁢                  ⁢          C      .      